(1) Field of the Invention
The present invention relates to an image processing apparatus, an image forming apparatus, and an image processing method, and especially to an edge enhancement processing technique.
(2) Description of the Related Art
In practice employed in an image forming apparatus such as a digital color copier, an original image obtaining by reading a document is subjected to so-called edge enhancement processing in order to increase the density of edge areas (for example, boundaries between a character image and a background image) before such an image is formed on a recording sheet. Such edge enhancement processing is carried out by an image processing apparatus installed in an image forming apparatus in order to sharpen boundaries and thus to improve readability of, for example, characters by widening the difference in density between edge areas and background.
In a conventional image processing apparatus, when a target pixel, i.e. a pixel currently subjected to processing, is judged to be a pixel in an edge area, the target pixel is enhanced by an enhancement amount that is determined in relation to pixels surrounding the target pixel within a predetermined area (hereinafter, this technique is referred to as “a first prior art technique”). Although this technique successfully sharpens boundaries, there is a problem as follows. That is, since the edge enhancement processing is to simply add an enhancement amount to each pixel in edge areas in an original image, when density variations originally exist in the edge areas, not only that the density variations remain even after the processing, but also that the density variations become even more noticeable.
To address the above problem, there has been another technique in which each pixel in edge areas is increased its density in a manner to reduce the difference in density with the surrounding pixels, and then an enhance amount as above is applied to each pixel (hereinafter, this technique is referred to as “a second prior art technique”).
Although better then the first prior art technique, according to the second prior art technique, density variations that exist in edge areas of an original image still remain noticeable to a considerable extent even after edge enhancement processing.